The invention relates to chiral or achiral liquid crystals, preferably with extended smectic C phases for liquid-crystalline mixtures for electrooptical arrangements for the modulation of the transmitted or reflected light, as well as for the colored or black-and-white display of numerals, symbols and moving or still pictures.
It is well known that liquid-crystalline substances can be used for the modulation of light as well for the display of measured values or for the reproduction of information. This is based on the fact that the preferred orientation of thin layers of the crystalline-liquid substances can be changed by the application of an electrical field. This change in the preferred orientation of crystalline-liquid substances is associated with a change in the optical behavior (birefringence, rotation capability, light absorption). Depending on the initial orientation, the dielectric and optical anisotropy, the conductivity, the spontaneous polarization and the dichroism, which are achieved by the special pretreatment of the electrodes or the addition of suitable substances, as well as on the strength, direction and frequency of the applied electrical field, different types of electrooptic effects are observed and used technically (M. Tobias: International Handbook of Liquid Crystal Displays 1975-76, Ovum Ltd., London; G. Meier, E., Sackmann, J. G. Grabmeyer: Applications of Liquid Crystals, Springer-Verlag Berlin-Heidelberg-New York 1975; N. A. Clark, S. T. Lagerwall: Appl. Phys. Lett. 36, 899 (1980)).
For example, a known method is based on the fact that, by the application of an electric field to a substance with an optically active smectic C phase, the average direction of the longitudinal axes of the molecules and twice the tilt angle, can be changed. The rapid switching over, which occurs at the same time, can be observed, for example, for incident light, with the help of polarization as a strong intensity change.
For the technical application of ferroelectric smectic C phases, the substances used must fulfill certain conditions, such as
extended, optically active smectic C phases
chemical stability and
a negative dielectric anisotropy.
Since mixtures are used almost exclusively for the technique, it is necessary to find mixture components which, on the one hand, have high transition temperatures of the C phase into other phases and, on the other, at lower temperatures, including the supercooled regions, have no additional liquid crystalline phases. In this connection, it is not absolutely essential that these components be optically active themselves. It is only necessary that, in mixtures with optically active S.sub.C phases or other optically active substances, they form optically active mixtures.